I. Field of the Invention
The present invention relates to networks of wireless transceivers that implement Carrier Interferometry (CI) and/or CI-based coding.
II. Description of the Related Art
A wideband signal, such as a direct-sequence CDMA (DS-CDMA) signal, transmitted in a multipath environment experiences a frequency-selective fade. If the duration of the data bits is smaller than the multipath delay, the received signal experiences inter-symbol interference resulting from delayed replicas of earlier bits arriving at the receiver.
Improved DS-CDMA systems use interference cancellation to increase capacity; however, the required signal-processing effort is proportional to at least the cube of the bandwidth. Furthermore, DS-CDMA is susceptible to near-far interference, and its long pseudo-noise (PN) codes require long acquisition times. For these reasons, Orthogonal Frequency Division Multiplexing (OFDM) has been merged with DS-CDMA.
In multicarrier CDMA (MC-CDMA), a spreading sequence is converted from serial to parallel. Each chip in the sequence modulates a different carrier frequency. Thus, the resulting signal has a PN-coded structure in the frequency domain, and the processing gain is equal to the number of carriers.
In multi-tone CDMA, or multicarrier DS-CDMA, the available spectrum is divided into a number of equal-width frequency bands used to transmit a narrowband direct-sequence waveform. In U.S. Pat. No. 5,504,775, binary CDMA code symbols are applied to individual carriers in an OFDM system. U.S. Pat. Nos. 5,521,937, 5,960,032, and 6,097,712 describe multicarrier DSSS systems having direct-sequence coding on each subcarrier.
U.S. Pat. No. 5,955,992, PCT Pat. Appl. No. PCT/US99/02838, and U.S. Pat. Pub. No. 2002034191 describe CI, which is a multicarrier protocol implemented with polyphase codes. These polyphase codes may be used for multiple access, spread spectrum, channel coding, or encryption, as described in U.S. Provisional Appl. No. 60/259,433, filed Dec. 31, 2000. Multiple carriers are redundantly modulated with data streams that are orthogonalized by virtue of different sets of phases encoding each data stream. Interferometry of the carriers provides the means to orthogonalize the data streams, whether the carriers are combined or processed separately. Weights applied to the carriers shape the carrier superpositions, thus, allowing CI signals to appear as single-carrier waveforms, such as Time Division Multiple Access (TDMA) or DS-CDMA signals.
Adaptive antenna arrays may be implemented with DS-CDMA communications to provide significant improvements in range extension, interference reduction, and system capacity. To identify a particular user, a DS-CDMA system demodulates Walsh codes after converting the received signal from analog radio frequency (RF) to digital. Therefore, an adaptive antenna array requires information about the user codes, or it needs to demodulate many different incoming RF signals to track mobile users. These methods are complex processes that are more difficult to implement than tracking users in non-CDMA systems. Furthermore, the wideband nature of DS-CDMA signals restricts the effectiveness of beam forming, interference nulling, spatial interferometry multiplexing, and other techniques employed by adaptive antenna arrays.
U.S. patent application entitled “CI Multiple Input, Multiple Output,” filed on Nov. 22, 2000, describes applications of multiple-input, multiple-output processing (such as antenna-array processing) to CI signals. CI processing allows wideband single-carrier signals, such as DS-CDMA transmissions, to be processed as a plurality of narrowband components. This simplifies array processing by facilitating beam forming, null steering, space-frequency processing, as well as other adaptive array processing techniques.
U.S. Pat. No. 4,901,307 introduces the concept of marginal isolation, which is another method of exploiting spatial division multiplexing in communication networks. Since cellular implementations of DS-CDMA are typically interference limited, even small reductions in the overall power level of the system allow for increased system capacity.
Ad-hoc networks allow subscriber units to perform base station functions. This allows micro-cell subnetworks, which lowers overall system power levels and improves spectral efficiency. U.S. Pat. No. 5,943,322 describes an adaptable DS-CDMA network that uses a control-signal channel to assign base-station functions (e.g., power control and synchronization) to a subscriber unit. U.S. Pat. No. 6,233,248 provides for multi-address transmissions to be sent on common paths between nodes until the paths diverge. U.S. Pat. No. 5,422,952 allows signals for a particular user to be transmitted throughout the entire network. Different transmissions are provided with unique PN codes.
None of the prior-art references describe the use of CI, CI-based protocols, or CI coding in ad-hoc networks. Thus, none of the prior-art references can provide the improved bandwidth efficiency, enhanced power efficiency, increased range, increased throughput, and superior immunity to multipath, jamming, and co-channel interference enabled by the combination of CI and adaptable networks.